Various chemical processes include a stream of gas at relatively high pressure that it also hot enough to make it desirable to recover heat from the gas stream and generate steam for further use in the process or for other uses within a plant. If the volume of the gas is high enough, however, it is impractical to use a fire tube boiler since its components, especially the outer shell, would have to be inordinately thick in order to withstand the mechanical stresses involved; this can also be a problem if high pressure steam is desired from the heat recovery boiler.
Moreover, a hot gas stream from which heat is to be recovered will often contain substantial amounts of solid particulates. This causes additional design problems for a heat recovery boiler because of the increased likelihood that the solids will accumulate on or about heat transfer components of a boiler and thereby substantially reduce its efficiency. This results in the need for periodic shutdown of the boiler for removal of the accumulated solids from the unit.
As one example, coal gassification processes often have a hot high pressure gas stream emanating from the gassification unit and have substantial requirements for steam. It is not unusual for coal gassification processes to have a gas stream at a pressure as high as 600 to 900 psig; also, the gas stream will contain quite substantial amounts of solids particulates. The gas stream is hot enough and steam requirements of the process are high enough to make it desirable to recover heat from the stream and use it to produce steam to meet the energy requirements of the process.
As another example, the so-called steam methane reforming process as employed in the production of methanol, ammonia, or hydrogen from natural gas can have gas streams with pressures as high as 300 to 500 psig from which it is desirable to recover heat for generation of process steam. Prior boiler designs for this purpose usually have been of either U-tube or bayonet tube designs. However, both of these types are prone to the accumulation of solids and difficult to clean when the accumulation reaches a level sufficient to significantly impair heat recovery.
The problems of prior heat recovery boiler designs that we are aware of for use in the foregoing processes were the impetus for the research and development work which resulted in our new boiler design described hereinafter. In addition, we have sought to provide a heat recovery boiler that can be effectively used to recover heat from gas streams of lower pressures, such as in the range of about 8 to 20 psig, as typically found in oil refining processes employing catalytic cracking.